1. Field of the Invention
The present invention relates to a method and apparatus for continuously quenching a steel plate by introducing the steel plate into a coolant vessel.
2. Description of the Prior Art
In order to increase the rate of cooling of a steel plate to effectively quench the steel plate, it is necessary to increase the relative velocity between the surface of the steel plate and a cooling water. To this end, a typical conventional cooling apparatus employs a cooling water jet for jetting pressurized cooling water from nozzles into collision with the steel plate surface.
This cooling apparatus, however, inconveniently increases the size of the quenching system as a whole because of the necessity of equipment for pressurizing the cooling water to a high pressure, and consumes massive cooling water uneconomically. In consequence, the production cost and running cost of the quenching system are raised undesirably. The increase of the cooling water consumption rate also requires a larger size of the cooling water pipes occupying large space, which in turn necessitates larger height and length of the quenching system as a whole requiring suitable reinforcement of the frame and the like for supporting the increased weight of the cooling water pipes and other internal structures. In addition, the increased size of the cooling water pipes is inevitably accompanied by an increase in the pitch of rows of the cooling pipes arranged in a side-by-side fashion, which in turn makes it difficult to increase the density of water jets and, hence, to obtain higher cooling effect. Furthermore, it has been often experienced that the cooling water nozzles are clogged to provide an uneven quenching effect. Once the clogging of the nozzles occur, much time and money are required for recovering the uniform distribution of water jets.
In order to overcome these problems, the present inventors have already proposed, in Japanese Patent Application No. 167102/1972 corresponding to the International Patent Application No. PCT/JP 81/00356, a continuous steel plate quenching apparatus having a water storage vessel through which the steel plate is made to run so as to be cooled by the cooling water, and impellers disposed in the vessel and adapted to displace the cooling water in the direction of running of the steel plate or, alternatively, in the direction opposite to the direction of running of the steel plate.
This continuous steel plate quenching apparatus will be explained hereinunder with specific reference to FIGS. 1 to 4.
FIG. 1 is a side elevational view of the continuous steel plate quenching apparatus proposed in the aforementioned Patent application, while FIGS. 2 and 3 are sectional views taken along the lines II--II and III--III of FIG. 1, respectively. The essential part of the apparatus is shown in a larger scale in FIG. 4.
Referring first to FIG. 1, the continuous steel plate quenching apparatus has a cooling water storage vessel 10 provided in its opposing walls with openings 14 for permitting a steel plate 12 to be quenched to pass therethrough. The vessel 10 accomodates rolls 16 and rolls 18 arranged in rows at the upper and lower sides of the path of the steel plate 12 and adapted to clamp the steel plate 12 from the upper and lower sides of the latter to feed the same in the direction of arrow C. The rolls 16 and 18 are adapted to be driven rotatingly in the directions of arrows A and B, respectively, by driving devices 28 and 30 (see FIG. 2). The cooling water storage vessel 10 is filled with cooling water 42 supplied through water supplying main pipes 64 and 66. A plurality of impellars 32, each having a plurality of blades 40 and an axis parallel to the rolls 16, are arranged in a row such that each impeller 32 takes position between each pair of adjacent rolls 32. Similarly, impellers 34 are disposed between adjacent rolls 18. The impellars 32 and 34 are adapted to be driven in the directions of arrows D and E, respectively, by driving devices 52 and 54 (see FIG. 3) so as to agitate the cooling water. In order to effect an efficient agitation of the cooling water, the impellers 32 and the impellers 34 are housed by respective housings 56 and 58 each of which are opened at its position facing the steel plate to be cooled.
The rolls 16 and 18, which are arranged in respective rows at a suitable pitch as shown in FIG. 1, are rotatably supported at their both ends by bearings 20 and 22, respectively, as will be best seen from FIG. 2. The axes of the rolls 16 and 18 are held horizontally and are extended in the breadthwise direction of the water storage vessel 10. One end of each of the rolls 16 and 18 are extended beyond the bearing 20 or 22 to the outside of the vessel 10. The extended ends are drivingly connected through couplings 24 and 26 to the aforementioned driving devices 28 and 30 so that the rolls 16 and 18 are driven by these driving devices. Namely, the rolls 16 of the upper row are rotatingly driven in the direction of arrow A while the rolls 18 of the lower row are made to rotate in the opposite direction as shown by arrow b, so that the steel plate 12 to be quenched, clamped between these rows of rolls, are driven continuously in the longitudinal direction of the water storage vessel 10.
An upper water supplying pipe 74 and a lower water supplying pipe 74 are disposed externally of the water storage vessel 10 in the vicinity of each opening 14, at the upper side and lower side of the passage of the steel plate 12. Each water supplying pipe 74 has a slit nozzle 76 directed toward the opening 1. In operation, pressurized water 78 is jetted from the slit nozzle 76 towards the opening 14 to form a water seal around the opening 14.
In this quenching system, the impeller housings are connected to respective main water supplying pipes 64,66 through branch pipes 60,62, so that the quenching system as a whole is made complicated and expensive although it is improved to some extent as compared with conventional systems. In addition, the cooling water tends to flow in the breadthwise direction of the steel plate 12 as indicated by arrows L,M and N in FIG. 2, so that the uniform cooling of the water in the breadthwise direction may fail. Furthermore, since all of the water supplied into the water storage vessel 10 overflows from the upper edges of said walls, an upward flow of water is formed in the water storage vessel 10 so that different cooling effects are developed on the upper surface and lower surface of the steel plate.
It is to be pointed out also that, as will be seen from FIG. 4, although the rotation of the impeller 32 produces the flow of cooling water, the flow of cooling water on the surface of the steel plate is only effected at the lower side of the impeller 32 and the water thereafter flows over the roll 16 in the direction indicated by arrow J, so that no substantial cooling effect is produced on the portion of the steel plate 12 just beneath the roll 16. Namely, the rolls 16 and 18, which are flat rolls, make line contact with the steel plate 12 along the length of these rolls, so that the flow of cooling water in the longitudinal direction of the water storage vessel, produced by each impeller, is blocked by the roll and is deflected upwardly, i.e. away from the surface of the steel plate 12, as indicated by arrow J. Therefore, the flow of water is weakened in the region where the steel plate 12 is contacted by the rolls 16 and 18, resulting in a lowered quenching efficiency.
The present inventors, through a series of experiment, found out that the cooling water stagnates around the base portions of the blades 40, i.e. around the juncture between the blades 40 and the shafts 36,38 of the impellers because the water in such region can hardly be mixed with the newly supplied cold water, although the replacement with newly supplied water takes place vigorously in the region around the radial outer ends of the blades. In consequence, the temperature of water stagnant around the shafts is rotated following up the rotation of the shafts 36,38 without making any contribution to the cooling of the steel plate. Furthermore, when air bubbles or voids which are happened to be contained by the supplied cooling water are introduced into the regions around the shafts 36,38 where the blades 40 are jointed to these shafts, the water containing the air bubbles or voids undesirably stagnates in these regions to make it difficult to discharge the air bubbles to the outside of the water storage vessel 10. This phenomenon inconveniently raises the temperature of the cooling water to seriously deteriorate the cooling performance. The air bubbles or voids stagnant around the base ends of the blades 40 cause various further problems such as reduction in efficiency of use of the cooling water, increase in the resistance against rotation of the blades and, hence, increase in the driving power, as well as corrosion of the impellers.
In this quenching system in which the water storage vessel is always filled with cooling water, a forced cooling is effected leaving the cooling power of so-called dip cooling, even when the impellers are not rotated. It is, therefore, rather difficult to apply this quenching system when the quenching is to be made in a controlled manner at a comparatively small cooling rate. On the other hand, a quenching system employing two-fluid jetting nozzles has been known in which jets of mixture of water and compressed air are applied to the material to be quenched in the air. In this cooling system, the rate of water jet and, therefore, the cooling power can be adjusted and controlled over a wide range so that it is possible to impart any desired mechanical properties to the steel in accordance with the use and kind of the steel, through suitably selecting the cooling rate. On the other hand, however, it is necessary to use a large quantity of compressed gas and to employ a source of cooling water of high pressure, in order to attain high quenching efficiency with this quenching system. This is quite disadvantageous from economical point of view and may incur a rise of the production cost.
In the aforementioned quenching system incorporating the impellers 32,34, only a small gap on an order of several tens of millimeters or less is left between the radially outer ends of the blades 40 and confronting surfaces of the steel plate 12, while the impellers are rotating during quenching. Therefore, when the steel plate 12 is happened to be warped or deflected during quenching, the blades 40 may be accidentally contacted by the blades 40 to cause breakdown of the impellers 32,34. It is conceivable even that the steel plate 12 cannot be passed through the clearance between the upper and lower impellers 32 and 34, due to an excessively large warp or deflection. The steel plate 12, which is accidentally contacted by the blades 40 of the impellers 32,34, is seriously damaged at the contacted surface or surfaces.
It is to be pointed out also that, in this quenching system, it is not possible to treat a steel plate of a thickness exceeding the thickness of the opening 14. To the contrary, a too small thickness of the steel plate makes it difficult to form the water seal around the opening 14.
In the quenching system of the type described, it is often required to pass a hot steel plate without effecting any quenching on the steel sheet. In such a case, the upper structures such as upper rolls 16, upper impellers 32, upper impeller housings 56 and so forth are shifted upwardly away from the passage of the hot steel plate thereby to prevent heating of these structures by the heat radiated from the hot steel plate, as well as the collision by the steel plate. In this state, the upper slit nozzles 76 are also moved upwardly apart from the passage of the hot steel plate. Under such a condition, the steel plate is conveyed solely by the lower rolls 18. Any excessive heating of the lower rolls 18 and the lower impellers 34 of the quenching system by the heat from the hot steel plate is prevented by filling the lower portion of the water storage vessel 10 up to such a level as not to reach the steel plate 12, i.e. below the pass line. The overheating of the lower slit nozzles 76 is also prevented by jetting cooling water through the lower slit nozzles at such a rate that the jets of water from the lower slit nozzles do not reach the steel plate. On the other hand, the upper structures such as upper rolls 16, upper impellers 32 and so forth are inevitably subjected to the heat radiated from the steel plate and, therefore, are heated to a temperature of 100.degree. C. or higher but thermal distorsion of these structures can be avoided by rotating them during passing of the hot steel plate. However, the upper slit nozzles 76, which also are subjected to the heat radiated from the hot steel plate, cannot effectively be cooled because they cannot be rotated nor supplied with cooling water unlike the lower slit nozzles 76. Namely, if the cooling water is jetted from the upper slit nozzles 76, the steel plate which is not intended for quenching is undesirably cooled by the cooling water to locally change its property resulting in a serious deterioration in the quality of the steel plate. Therefore, in the known quenching system of the type described, when the hot steel plate which is not to be quenched is made to pass through the quenching line, the upper slit nozzles 76 are excessively heated to high temperature by the heat radiated from the steel plate to make thermal distorsion resulting in a change in the pitch of the upper slit nozzles or alternatively the nozzles as a whole are bent or deformed. This inconveniently causes a disturbance in the pattern of jet of the cooling water when the water jet is started for the next quenching operation. With such a disturbed water jet pattern, it is not possible at all to attain the expected quenching effect.